Pixel structures, as used in image sensors and other sensor applications, have a finite dynamic range. The dynamic range is the range, typically expressed as a ratio, between the largest intensity value that can be resolved by the pixel and the smallest intensity value that can be resolved by the pixel. When used in image sensors, such as CMOS active pixel image sensors, this limits the ratio between the brightest and darkest image values that can be resolved by the image sensor. The smallest intensity value is limited by the read noise of the pixel. The largest intensity value is limited by the amount of charges that can be stored inside the pixel and effectively read out.
Various techniques have been proposed to extend the dynamic range. A summary of various high dynamic range techniques is given in the article “Wide-Dynamic-Range CMOS Image Sensors—Comparative Performance Analysis”, A. Spivak et al, IEEE trans. El. Dev, vol. 56, No. 11, pp. 2446, November 2009. Techniques have been used which compress the photosignal in a piecewise linear way by partial reset (e.g. U.S. Pat. No. 7,106,373) or by charge division during readout (e.g. DE69805555). In such pixels, colour reconstruction is difficult since it is not exactly known which slope is used close to the kneepoints of the image. Another disadvantage of such pixels is that the exposure period is different for different parts of the response curve. Pixels in bright areas have received a shorter exposure period than pixels in a dark area. This results in distortion artefacts in moving images.
Other techniques have been proposed for increased dynamic range, like pixels with time-to-saturation counters, but they result in much more complex pixel implementations. One technique to achieve a high dynamic range uses different gain in the readout path (e.g. B. Fowler, “Wide Dynamic Range Low Light Level CMOS Image Sensor”, proc. International Image Sensor Workshop, Bergen, June 2009). The photodiode signal is read out through a single floating diffusion, which must be designed to handle the maximum charge packet that is read out from the pixel. This method only solves the dynamic range limitations in the readout chain outside of the pixel array.
“Non-Linearity in Wide Dynamic Range CMOS Image Sensors Utilizing a Partial Charge Transfer Technique”, Suhaidi Shafie et al, Sensors 2009, vol. 9, p. 9452-9467 describes another technique of partial charge transfer from the photodiode to the floating diffusion by multiple transfers through the same transfer gate. The gate voltage bias is modulated to control the maximum amount of charge transferred in each cycle. This requires subsequent readings from the pixel, which results in more time required to read the pixel data. Furthermore, the voltage control of the transfer gate is important, and threshold voltage variations on transfer gates will cause pixel-to-pixel non-uniformities in the different transfers.
The present invention seeks to provide an alternative way of improving the dynamic range of a pixel structure.